How can I get more power for freeway driving from my 3.9 V8 - Defender Source
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  #1  
Old December 7th, 2014, 09:35 AM
Jcwhancock
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How can I get more power for freeway driving from my 3.9 V8

Love my D90 with the original 3.9 V8 with 5 speed but would appreciate any suggestions on how to increase power at higher mph when on the freeway. I'm fed up slipping back on the hills and sliding down from 65 mph down to 50 mph on hills. Any suggestions without replacing the engine ?
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  #2  
Old December 7th, 2014, 09:45 AM
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These motors are anemic...options are really limited. Check your air filter to make sure it's not past it's prime, but other than that you're swapping to a 4.6 or buying a jeep .

What transfer case do you have in it? Make sure someone didn't swap it out for a lower ratio than the 1.4 that came with it. In theory you could also change your diff gearing I suppose to something like 4.11s from the 3.54s, but seems like an expensive exercise to just get a little more power.
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  #3  
Old December 7th, 2014, 09:47 AM
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Quote:
Originally Posted by Jcwhancock View Post
Love my D90 with the original 3.9 V8 with 5 speed but would appreciate any suggestions on how to increase power at higher mph when on the freeway. I'm fed up slipping back on the hills and sliding down from 65 mph down to 50 mph on hills. Any suggestions without replacing the engine ?
Make sure the engine is well tuned up. In normal configure it should keep up on the hills ... Over the years lots of items get messed about and the engine can seem to be running ok but really it's not.

I know this problem all too well!
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Old December 7th, 2014, 09:52 AM
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Convert to a 1.22 transfer case, we did a 93 110 with a 1.22 that did 70 at 2300rpms
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Old December 7th, 2014, 10:04 AM
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Quote:
Originally Posted by Brewie212 View Post
Convert to a 1.22 transfer case, we did a 93 110 with a 1.22 that did 70 at 2300rpms
It should already have a 1.22. If anything, a lower ratio would help it on the highway as the Rover V8 doesn't make any real power until higher RPMs.

Don't be afraid to push the motor hard. You can shift it into fourth and floor it with no consequence.
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Old December 7th, 2014, 11:22 AM
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I believe as we've all experienced with that particular engine, it's the hills that kill you. I can reach 85 on the flats in mine but you give me a little hill I'll be lucky to stay up to 65.
I think 4.6 is the only thing that could. Or just learn to pose smile & wave as the grandmas pass you by in their 78 impalas
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Old December 7th, 2014, 11:37 AM
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you could do this.

GM Gen 4 conversion including LS. Sorry I don't know how to post the link.
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Old December 7th, 2014, 11:38 AM
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Can you do that conversion & keep it manual?
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Old December 7th, 2014, 11:52 AM
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It's a long thread, good read though. I think that they're trying to come up with a manual option. I'm not 100% certain though.
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Old December 7th, 2014, 12:01 PM
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Yeah I read it a while back. It didn't seem possible up to where last I read.
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Old December 7th, 2014, 12:02 PM
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Supercharger.
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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Old December 7th, 2014, 12:18 PM
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You can do Nicks Ls conversion with an nv4500 5 speed manual.
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Old December 7th, 2014, 01:33 PM
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Old December 7th, 2014, 06:01 PM
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Mine will run at 70 all day.. I don't mind just cruising in the right lane for trips.
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Old December 7th, 2014, 06:28 PM
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When my Disco had the 4.0, it had a difficult time going up steep grades. Of course now it has the HS2.8. Problem solved.
The RR has the 4.6 and it has no problem going up that hill.
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Old December 7th, 2014, 06:56 PM
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You have to start with basics like a compression test and a vacuum test to determine if your existing engine is tired. A valve job and fresh cam typically make a huge difference in waking up a tired engine.
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Old December 7th, 2014, 07:18 PM
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who's done it?

Quote:
Originally Posted by leastonce View Post
Supercharger.


Anyone supercharged or turbo'd a 3.5 or 3.9? I could see someone pulling everything from a RR but it would seem easier to use the RR engine as well.
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Old December 7th, 2014, 07:23 PM
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Anyone supercharged or turbo'd a 3.5 or 3.9? I could see someone pulling everything from a RR but it would seem easier to use the RR engine as well.
They are rare, and from what I understand, not very reliable. You can probably get more power from a 4.6.

A pretty cheap upgrade is to replace the cylinder heads with the newer style that only have 10 bolts. That should yield a higher compression ratio. Not really worth it unless your head gaskets are already leaking.
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  #19  
Old December 7th, 2014, 10:02 PM
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The easiest gain for about 7bhp at the wheels is to leave to air cleaner trumpet off... Yes I found this out after accidentally leaving the trumpet off on a dyno run. it does make a slight but noticeable improvement.

My reason for the dyno run was to set a baseline before a 4.6 conversion with a stage 2 cam, warmed over heads and other stuff - that gave a 45bhp improvement on the dyno and the ability to get up Aspen hill in 4th gear instead of 2nd and third!

The standard engine is limp unfortunately and nothing really fixes that without swapping in a 4.6. - but then you fight overheating flogging it up the hills in hot summers!
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Old December 7th, 2014, 10:27 PM
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A few things.

You do not want a turbocharger on a Rover V8. Will fail.

The 3.5 does seem to like to be stepped on. I live on a hill, and I've been disappointed by the 3.5 in the 110. Even my stock Series IIa will take the hill in 3rd gear when empty. But the 110 would not do it at all. Had to shift down to 2nd about 100 yards early from where the IIa usually does.

But yesterday I just decided to screw it, gave it a bit of a run up for the hill, not much, just 30mph instead of 25. Then really goosed it in 3rd gear when the hill got steep where I usually have to shift to 2nd. It just growled at me and kept pulling. Went all the way up in 3rd.

Yes, the 3.5 is weak, but with KN filters on instead of that steampunk looking airfilter they had on it stock, it will do the job for a while.

I still don't really know if I will be able to get a Tdi in it. But I'm certainly going for a 4.2 or better.
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